Control techniques for motor driven systems

ABSTRACT

Embodiments of the present invention provide a drive signal for a motor-driven mechanical system whose frequency distribution has zero (or near zero) energy at the expected resonant frequency of the mechanical system. The drive signal may be provided as a pair of steps sufficient to activate movement of the mechanical system and then park the mechanical system at a destination position. The steps are spaced in time so as to have substantially zero energy at an expected resonant frequency f R  of the mechanical system. The drive signal may be filtered to broaden a zero-energy notch at the expected resonant frequency f R .

RELATED APPLICATION

This application is related to co-pending application “Controltechniques for motor driven systems”, reference number 13641-387302,also filed on Feb. 9, 2009.

BACKGROUND

The present invention relates to motor control and control of motordriven systems. In particular, it relates to control of motor drivensystems that minimize ringing or ‘bounce’ in the mechanical systems thatare under motor control.

Motor driven translational systems are commonplace in modern electricaldevices. They are used when it is necessary to move a mechanical systemwithin a predetermined range of motion under electrical control. Commonexamples can include autofocus systems for digital cameras, videorecorders and portable devices having such functionality (e.g., mobilephones, personal digital assistants and hand-held gaming systems). Insuch systems, a motor driver integrated circuit generates a multi-valuedrive signal to a motor which, in turn, drives a mechanical system (e.g.a lens assembly, in the case of an auto-focus system). The motor drivergenerates the drive signal in response to an externally suppliedcodeword. The code word often is a digital value that identifies alocation within the mechanical system's range of motion to which themotor should move the mechanical system. Thus, the range of motion isdivided into a predetermined number of addressable locations (called“points” herein) according to the number of code words allocated to therange of motion. The drive signal is an electrical signal that isapplied directly to the motor to cause the mechanical system to move asrequired.

Although the types and configurations of the mechanical systemstypically vary, many mechanical systems can be modeled as a mass coupledto a spring. When a motor moves the mass according to the drive signal,the motion generates other forces within the system which can cause themass to oscillate around the new location at some resonant frequency(f_(R)). For example, resonant frequencies of approximately 110 Hz havebeen observed in consumer electronic products. Such oscillationtypically diminishes over time but it can impair performance of thedevice in its intended function by, for example, extending the amount oftime that a camera lens system takes to focus an image.

FIG. 1 is a simplified block diagram of a motor-driven system commonlyused in lens drivers. The system includes an imaging chip 110, a motordriver 120, a voice coil motor 130 and a lens 140. The motor drivergenerates a drive signal to the voice coil motor in response to a codeprovided by the imaging chip. In turn, the voice coil motor moves thelens within its range of motion. Movement of the lens changes the waythe lens focuses incoming light on a surface of the imaging chip, whichcan be detected and used to generate new codes to the motor driver. FIG.2 is a frequency plot of possible response of the system of FIG. 1,illustrating a resonant frequency at frequency f_(R).

FIG. 3 illustrates two drive signals generated by conventional motordrivers. A first drive signal is a step function, that changes from afirst state to a second state as a discontinuous jump (FIG. 3( a)). Thesecond illustrated drive signal is a ramp function that changes from thefirst state to the second state at a fixed rate of change (FIG. 3( b)).Both types of drive signals, however, cause the ringing behavior thatimpairs performance as noted above. FIG. 4, for example, illustratesringing observed in one such mechanical system.

The inventors have observed that the ringing behavior of suchmotor-driven systems unnecessarily extends the settling times of suchmechanical systems and degrades performance. Accordingly, there is aneed in the art for such motor-driven systems that can be drivenaccording to a digital codeword and avoids the oscillatory behaviornoted in these systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary mechanical system suitable foruse with the present invention.

FIG. 2 is a graph of frequency response of an exemplary mechanicalsystem and oscillation that may occur during activation.

FIG. 3 illustrates conventional drive signals for mechanical systems.

FIG. 4 illustrates response of a mechanical system observed under aunitary step drive signal.

FIG. 5 illustrates a drive signal according to an embodiment of thepresent invention.

FIG. 6 is a graph illustrating height and position of steps of the drivesignal of FIG. 5.

FIG. 7 is a graph illustrating energy distribution by frequency of adrive signal of the present invention.

FIG. 8 illustrates response of a mechanical system observed under adrive signal such as shown in FIG. 5.

FIG. 9 is a block diagram of a system according to an embodiment of thepresent invention.

FIG. 10 is a graph illustrating energy distribution by frequency ofanother drive signal of the present invention.

FIG. 11 is a block diagram of a system according to an embodiment of thepresent invention.

FIG. 12 is a block diagram of another mechanical system suitable for usewith the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention provide a drive signal for amotor-driven mechanical system whose frequency distribution has zero (ornear zero) energy at the expected resonant frequency of the mechanicalsystem. The drive signal may be provided as a pair of steps sufficientto activate movement of the mechanical system and then park themechanical system at a destination position. The steps are spaced intime so as to have substantially zero energy at an expected resonantfrequency f_(R) of the mechanical system. The drive signal may befiltered to broaden a zero-energy notch at the expected resonantfrequency f_(R).

FIG. 5 is a graph illustrating an exemplary drive signal according to anembodiment of the present invention. The drive signal is a multi-stagestep function that changes at times corresponding to a time constant:

$\begin{matrix}{t_{c} \cong {\frac{1}{2\; f_{R}}.}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$This translates to a drive signal with two steps, a first step at timet₀, having an amplitude corresponding to one half the level needed totraverse a distance separating a old position (P_(OLD)) from a newposition (P_(NEW)) (ΔP=P_(NEW)−P_(OLD)). A second step may occur at timet₀+t_(c), also having an amplitude corresponding to ½ΔP. FIG. 6illustrates differential response of the drive signal of FIG. 5.

FIG. 7 is a graph illustrating energy distribution of the drive signalof FIG. 5 by frequency. As shown, the drive signal has non-zero energydistribution at frequencies both above and below the resonant frequencyf_(R). At the resonant frequency fR, the drive signal has zero energy.This energy distribution minimizes energy imparted to the mechanicalsystem and, therefore, avoids oscillation that may occur in suchsystems.

FIG. 7 also illustrates energy distribution that may occur in a drivesignal generated from a unitary step function (phantom). In this graph,the system has non-zero energy at the resonant frequency f_(R), whichcauses energy to be imparted to the mechanical system at this frequency.This non-zero energy component at the resonant frequency f_(R) isbelieved to contribute to the prolonged oscillation effect observed bythe inventors.

FIG. 8 is a graph that illustrates response of a mechanical system whendriven by a drive signal having a shape as shown in FIG. 5 (case (a)).The mechanical system starts at a position P_(OLD) and moves to aposition P_(NEW). Activation pulses are applied at times t₀ andt₀+t_(C). In this example, P_(OLD) corresponds to 27 μm (digital code50) and P_(NEW) correspond to 170 μm (digital code 295), t₀ correspondsto t=0 and t_(C) corresponds to 3.7 ms.

FIG. 8 compares the mechanical system's response under the drive signalproposed herein (case (a)) against the response observed when driven bya drive signal according to a unitary step function (case (b)). Whereasin case (a) the mechanical system has settled on the new positionP_(NEW) after about 4 ms, the same mechanical system exhibits prolongedoscillation in case (b). Even after 30 ms, the mechanical systemcontinues to oscillate about the P_(NEW) position. Accordingly, thedrive signal of FIG. 5 provides substantially faster settling times thanconventional drive signals.

FIG. 9 is a block diagram of a system 900 according to an embodiment ofthe present invention. As shown, the system may include registers910-930 for storage of data representing the old and new positions andthe expected resonant frequency of the mechanical system. The system 900may include a subtractor 940 to calculate ΔP from P_(NEW) and P_(OLD).The system 900 further may include an step generator 950 that receives asystem clock and generates pulses to an accumulator 960 according totiming determined from Eq. 1. The step generator 950 may generatepulses, for example, as shown in FIG. 6, having amplitudes eachcorresponding to one half the total distance to be traversed by themechanical system. The accumulator 960 may sum the aggregate value ofpulses generated by the step generator 950 and output the aggregatevalue to a multiplier 970 that also receives the ΔP value from thesubtractor. Thus, the multiplier 970 generates a signal corresponding tothe multi-step increments show in FIG. 5. The output of the multiplier970 may be input to an adder 980 that also receives the P_(OLD) valuefrom register 910. Thus, the adder 980 may generate a time varyingoutput signal sufficient to drive a mechanical system from a firstposition to a second position with minimal settling time.

When the mechanical system completes its translation from the oldposition to the new position, the old position may be updated. In thesystem illustrated in FIG. 9, after the step generator 950 generates itsfinal step to the accumulator, it also may generate a transfer signal toregisters 910 and 920 to cause the old position register 910 to beupdated with data from the new position register 920.

The drive signal of FIG. 5 works well if the resonant frequency f_(R) ofthe mechanical system matches the ‘notch’ of the drive signal precisely(e.g. within ±3%). Unfortunately, system manufacturers often do not knowthe resonant frequency of their mechanical systems precisely. Moreover,particularly in consumer devices where system components must be madeinexpensively, the resonant frequency can vary across differentmanufacturing lots of a common product. Thus, although a motor drivermight be designed to provide a notch at an expected resonant frequencyf_(RE), there can be a substantial difference between the expectedresonant frequency and the actual resonant frequency of the mechanicalsystem (f_(RM)).

To accommodate such uses, the principles of the present invention may beexpanded to expand the frequency notch to allow greater tolerance in theresonant frequencies used with such systems. One such expansion includesproviding multiple layers of filtering to ‘widen’ the notch. FIG. 10 isa graph showing the expected effects of multiple layers of filtering.Four such layers of filtering are illustrated. Each such additionallayer of filtering expands a “notch” of frequencies for which there iszero energy imparted to the system. Although each layer of filteringdiminishes the aggregate amount of energy imparted to the system and,therefore, may cause slower movement by the mechanical system, suchfiltering may be advantageous to overall system operation by reducingsettling times for mechanical systems when the resonant frequency ofsuch systems cannot be predicted with precision.

FIG. 11 illustrates a simplified block diagram of a system 1100according to another embodiment of the present invention. The systemincludes a drive signal generator 1110 and one or more notch limitfilters 1120.1-1120.N provided in series. A first filter 1120.1 in thesystem 1100 may accept a drive signal from the drive signal generator1110. Each of the N filters (N≧1) may filter its input signal at anexpected resonant frequency (f_(RE)). Because the filters are providedin cascade, the multiple filters may operate collectively to provide afiltered drive signal having a notch that is wider than would occur froma single filter system.

The principles of the present invention find application in a variety ofelectrically-controlled mechanical systems. As discussed above, they maybe used to control lens assemblies in auto-focus applications forcameras and video recorders such as shown in FIG. 1. It is expected thatsystems using the drive signals discussed herein will achieve improvedperformance because the lens assemblies will settle at new locationsfaster than may occur in systems with conventional drive signals.Accordingly, cameras and video recorders will generate focused imagedata faster than previously achieved.

FIG. 12 illustrates another system 1200 according to an embodiment ofthe present invention. The system 1200 of FIG. 12 illustrates a lenscontrol system with multiple dimensions of movement. This system, aswith FIG. 1, may include an imaging chip 1210, a motor driver 1220,various motors 1230-1250 and a lens 1260. Each motor 1230-1250 may drivethe lens in a multi-dimensional space. For example, as shown in FIG. 12,an auto focus motor 1230 may move the lens laterally with respect to theimaging chip 1210, which causes light to be focused in a light-sensitivesurface 1210.1 of the chip 1210. A pitch motor 1240 may rotate the lensthrough a first rotational axis to control orientation of the lens 1260in a first spatial dimension. A yaw motor 1250 may rotate the lensthrough a second rotational axis, perpendicular to the first rotationalaxis, to control orientation of the lens 1260 in another spatialdimension.

In the embodiment of FIG. 12, the imaging chip 1210 may includeprocessing units to perform auto-focus control 1210.1, motion detections1210.2 and optical image stabilization (OIS) 1210.3. These units maygenerate codewords for each of the drive motors 1230-1250, which may beoutput to the motor driver 1220 on an output line. In the embodimentshown in FIG. 12, the codewords may be output to the motor driver 1220in a multiplexed fashion. The motor driver 1220 may include motor driveunits 1220.1-1220.3 to generate analog drive signals for each of thedrive motors 1230-1250. The analog drive signals may be generatedaccording to the foregoing embodiments discussed in connection withFIGS. 5-11. As with the case of a one dimensional lens driver, it isexpected that a multi-dimensional lens driver that is driven as shown inthe foregoing embodiments will achieve faster settling times than lensdrivers driven according to conventional drive signals.

Several embodiments of the present invention are specificallyillustrated and described herein. However, it will be appreciated thatmodifications and variations of the present invention are covered by theabove teachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention. Forexample, although the foregoing embodiment have presented a motor driversystem that responds to codewords to address one of severalindependently addressable positions, it is possible to employ theprinciples of the present invention in systems in which there are feweraddressable positions. For example, mechanical systems that togglebetween one of two different predetermined positions can avoid the needfor codewords. In such a case, drive signals may be generated simplyfrom binary activation signals. Additionally, it will be appreciatedthat the signal illustrated in FIG. 5 represents an idealized form of adrive signal with instantaneous response; in practice, some amount ofslew can be expected from a motor driver in actual operating conditions.Such effects have been omitted from the foregoing discussion so as notto obscure the principles of the present invention.

1. A method of driving a motor-driven mechanical system, comprising:responsive to a codeword identifying a destination position of themechanical system, generating a stepped drive signal having an amplitudesufficient to move the mechanical system from a start position to thedestination position, applying a first drive signal step to a motor ofthe mechanical system, applying a second drive signal step to the motorat a time t_(C) offset from the first step by:$t_{c} \cong \frac{1}{2\; f_{R}}$ where f_(R) is an expected resonantfrequency of the mechanical system.
 2. The method of claim 1, whereinthe stepped drive signal has substantially zero energy at the expectedresonant frequency.
 3. The method of claim 1, wherein the second stepparks the mechanical system at the destination position withsubstantially no oscillation.
 4. The method of claim 1, wherein thecodeword is generated by an image signal processor and the drive signalis applied to a lens drive motor.
 5. The method of claim 1, wherein themechanical system is a lens system having a multi-dimensional range ofmotion, an image signal processor generates codewords corresponding toeach dimension and the method generates multiple drive signals, onecorresponding to each of the codewords.
 6. The method of claim 5,wherein there are three dimensions and three codewords, one for lateralmovement of the lens system, one for pitch of the lens system and onefor yaw of the lens system.
 7. A method of driving a motor-drivenmechanical system, comprising: responsive to a codeword identifying adestination position of the mechanical system, determining a drivesignal amplitude corresponding to a difference in position (AP) from astart position to the destination position, applying a first drivesignal step to a motor of the mechanical system at half the drive signalamplitude, applying a second drive signal step to the motor at half thedrive signal amplitude, the second step being applied at a time t_(C)offset from the first step by: $t_{c} \cong \frac{1}{2\; f_{R}}$ wheref_(R) is an expected resonant frequency of the mechanical system.
 8. Themethod of claim 7, wherein the stepped drive signal has substantiallyzero energy at the expected resonant frequency.
 9. The method of claim7, wherein the second step parks the mechanical system at thedestination position with substantially no oscillation.
 10. The methodof claim 7, wherein the codeword is generated by an image signalprocessor and the drive signal is applied to a lens drive motor.
 11. Themethod of claim 7, wherein the mechanical system is a lens system havinga multi-dimensional range of motion, an image signal processor generatescodewords corresponding to each dimension and the method generatesmultiple drive signals, one corresponding to each of the codewords. 12.The method of claim 11, wherein there are three dimensions and threecodewords, one for lateral movement of the lens system, one for pitch ofthe lens system and one for yaw of the lens system.
 13. A method ofdriving a motor-driven mechanical system, comprising: responsive to anactivation signal, generating a stepped drive signal having an amplitudesufficient to move the mechanical system from a start position to thedestination position, applying a first step of the drive signal to amotor of the mechanical system, applying a second drive signal step tothe motor at a time t_(C) offset from the first step by:$t_{c} \cong \frac{1}{2\; f_{R}}$ where f_(R) is an expected resonantfrequency of the mechanical system.
 14. The method of claim 13, whereinthe mechanical system remains in a default position in the absence ofthe activation signal and moves to the destination position when theactivation signal is engaged.